Posttraumatic epilepsy (PTE) is a frequent consequence of traumatic brain injury (TBI) in both the civilian and military population. Although some investigations have been conducted to evaluate the pathophysiology of this clinical syndrome, no proven therapies have been validated in the human PTE population. Our recent findings demonstrate that moderate fluid percussion brain injury in rats leads to reduced seizure threshold weeks to months after trauma. This alteration in neuronal and circuit excitation is supported by behavioral, electrophysiological and histopathological data. In addition, preliminary findings summarized in this grant proposal demonstrate that modest posttraumatic hypothermia (33oC) induced 30 minutes after TBI increases seizure threshold, reduces neuronal vulnerability and aberrant axonal sprouting in the hippocampus. Therefore, the present proposal will build on these exciting findings by investigating receptor-dependent mechanisms of action on the development of seizure susceptibility after TBI and will for the first time utilize a combination therapeutic approach including modest hypothermia and neurotransmitter receptor directed therapy to improve traumatic outcome and reduce the incidence of PTE. In Specific Aim 1, the impact of injury severity on subacute (1 and 4 weeks) and chronic (12 wks and 1 yr) seizure susceptibility using clinically relevant outcome measures including electrophysiology and cognitive assessment will be determined. Injury severity has been associated with the clinical manifestation of PTE but there currently are no experiments directly addressing this issue. In this series of studies, a subthreshold dose of the seizure-inducing agent pentylenetetrazole (PTZ) will be given at several post-traumatic time points to determine if injury severity- dependent changes in seizure threshold occur. In Specific Aim 2, evidence for alterations in local hippocampal circuit activity after trauma due to an imbalance in inhibitory (GABAA) and excitatory (NR2B) receptor function will be assessed. These studies will determine if changes in receptor localization seen after status epilepticus influence the development of seizure susceptibility after TBI. In addition, patterns of neuronal vulnerability in hippocampal regions as well as evidence for mossy fiber sprouting will be conducted to determine if these traumatic consequences can be correlated with changes in seizure threshold. Quantitative immunocytochemical, western blotting and in situ hybridization approaches for regional and cellular protein and mRNA assessment will be performed to determine these injury severity-dependent changes. Finally, in Specific Aim 3, a novel combination treatment strategy including modest posttraumatic hypothermia combined with a NR2B receptor blocker (Ro 25,6981) will be tested. The benefits of modest hypothermia are well documented after CNS injury but are unknown for PTE. Therefore, this combination approach may be the key to providing pre-clinical efficacy data in treating PTE. The benefits and limitations of single treatment paradigms will also be directly compared to this combination approach to assess therapeutic window, dose response as well as therapeutic efficacy. Based on preliminary findings, this combination approach should provide protection against neuronal drop-out, receptor vulnerability as well as behavioral outcome measures including seizure activity and cognitive function. The proposed studies will be performed by a group of experienced investigators that have a proven track record in the pathophysiology and treatment of TBI. Data from these studies could potentially be used to support clinical trials targeting PTE in this vulnerable patient population.